[fix][broker] Fix NPE while publishing Metadata-Event with not init producer

[rdhabalia]

Motivation

Currently, if broker tries to publish metadata-event before Metadata Event synchronizer is successfully created producer then it throws below NullPointerException and it should be fixed

java.util.concurrent.CompletionException: java.lang.NullPointerException: Cannot invoke "org.apache.pulsar.client.api.Producer.newMessage()" because "this.producer" is null
        at java.base/java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:332) ~[?:?]
        at java.base/java.util.concurrent.CompletableFuture.uniApplyNow(CompletableFuture.java:674) ~[?:?]
        at java.base/java.util.concurrent.CompletableFuture.uniApplyStage(CompletableFuture.java:662) ~[?:?]
        at java.base/java.util.concurrent.CompletableFuture.thenApply(CompletableFuture.java:2168) ~[?:?]
        at org.apache.pulsar.broker.resources.BaseResources.setAsync(BaseResources.java:151) ~[org.apache.pulsar-pulsar-broker-common-4.jar:4]
        at org.apache.pulsar.broker.resources.NamespaceResources.setPoliciesAsync(NamespaceResources.java:145) ~[org.apache.pulsar-pulsar-broker-common-4.jar:4]
        at org.apache.pulsar.broker.admin.impl.NamespacesBase.setBacklogQuotaAsync(NamespacesBase.java:1343) ~[org.apache.pulsar-pulsar-broker-4.jar:4]
        at org.apache.pulsar.broker.admin.impl.NamespacesBase.lambda$internalSetBacklogQuota$276(NamespacesBase.java:2653) ~[org.apache.pulsar-pulsar-broker-4.jar:4]
        at java.base/java.util.concurrent.CompletableFuture.uniComposeStage(CompletableFuture.java:1187) ~[?:?]
        at java.base/java.util.concurrent.CompletableFuture.thenCompose(CompletableFuture.java:2309) ~[?:?]
        at org.apache.pulsar.broker.admin.impl.NamespacesBase.internalSetBacklogQuota(NamespacesBase.java:2653) ~[org.apache.pulsar-pulsar-broker-4.jar:4]
        at org.apache.pulsar.broker.admin.v1.Namespaces.setBacklogQuota(Namespaces.java:1171) ~[org.apache.pulsar-pulsar-broker-4.jar:4]

Modifications

This PR fixes NPE while publishing metadata event.

Verifying this change

• Make sure that the change passes the CI checks.

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This change is a trivial rework / code cleanup without any test coverage.

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This change is already covered by existing tests, such as (please describe tests).

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This change added tests and can be verified as follows:

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• Added integration tests for end-to-end deployment with large payloads (10MB)
• Extended integration test for recovery after broker failure

Does this pull request potentially affect one of the following parts:

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• Dependencies (add or upgrade a dependency)
• The public API
• The schema
• The default values of configurations
• The threading model
• The binary protocol
• The REST endpoints
• The admin CLI options
• The metrics
• Anything that affects deployment

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